The present invention relates to new toughened thermoset compositions.
The present invention also relates to new compositions for toughening thermoset compositions. The present invention also relates to processes for preparing new tougheners for thermoset compositions. The present invention also relates to composite materials and articles which contain the new toughened thermoset compositions.
Many advanced composite materials are used in high-performance structural materials which have high heat resistance. These materials find wide use in the construction, electronic, automotive, computer, aerospace, and electrical industries. Many of these advanced composite materials are based on the thermal curing (xe2x80x9cthermosettingxe2x80x9d) of liquid resin formulations. These liquid resin formulations often contain various components for forming rigid highly crosslinked polymeric matrices.
Unfortunately, it is well known that rigid highly crosslinked polymeric matrices are brittle and have poor impact strength. Various toughening agents have been developed over the years for toughening thermoset materials. Functional toughening agents have been incorporated in thermoset materials as small rubbery particles. Presently, small rubbery particles are incorporated in thermosets either by premixing rubber particles (e.g., core-shell polymer particles) into the thermoset liquid resin prior to curing, or by formation of rubbery microdomains upon curing of the thermoset liquid formulation.
One of the most important type of toughening agents that is widely used in thermosetting structural materials is the class of Liquid Rubbers (xe2x80x9cLRxe2x80x9d). LRs that are commonly used for toughening thermoset resins have low viscosities and tend to be miscible in the uncured liquid resin formulations. The LRs typically phase separate upon curing (crosslinking) of the thermoset resins to form rubbery microdomains in the crosslinked polymeric matrix of the thermosetting resin. These rubbery microdomains help to toughen the rigid crosslinked polymeric matrix while maintaining heat resistance and dimensional stability of the matrix. Various types of LRs are disclosed in Mulhaupt, R., xe2x80x9cFlexibility or Toughness?xe2x80x94The Design of Thermoset Toughening Agentsxe2x80x9d Chimia 44 (1990), pp. 43-52.
An important design parameter of a LR for toughening thermoset resins is its molecular weight. While phase separation and toughness typically improve with increasing molecular weight of the LR, compatibility between the LR and the uncured liquid thermoset resins typically improves with decreasing molecular weight. Ideally, the LR is miscible (i.e. forms a single phase) in the uncured liquid thermoset resin because single phase liquid thermoset resin formulations have lower viscosities than multi-phase liquid thermoset resin formulations. Because multi-phase liquids tend to exhibit complex rheological behavior compared to single-phase liquids, miscible thermosetting resin formulations tend to have better processing characteristics than immiscible, multi-phase, liquid thermoset resin formulations.
Most, if not all, known LRs for toughening thermoset resins contain functional groups. It is generally believed that these functional groups are required to enhance the interfacial adhesion of the phase separated rubbery domain to the crosslinked polymeric matrix. Often this interfacial adhesion is enhanced by covalent chemical bonding between the functional groups of the LRs and the functional groups of the crosslinkable polymer resin. Often the functional groups of the LRs are located at the ends of polymer chains, denoted xe2x80x9cterminally functionalxe2x80x9d or xe2x80x9cfunctionally terminatedxe2x80x9d LRs.
Commercially-available functionally terminated LRs include carboxy-terminated copolymers of butadiene and acrylonitrile monomers, known as xe2x80x9cCTBNxe2x80x9d resins, and amino-terminated copolymers of butadiene and acrylonitrile monomers, known as xe2x80x9cATBNxe2x80x9d resins. Similar copolymers end-functionalized with vinyl groups and epoxy groups are also known as xe2x80x9cVTBNxe2x80x9d and xe2x80x9cETBNxe2x80x9d, respectively. It is known that the carboxylic acid and amine functional groups of these LRs enhance their miscibility in uncured epoxy resins. In addition, their terminal functional groups tend to increase the molecular weight of the polymer chains in the rubbery microdomains during curing, which also tends to improve impact strength.
Of the two common thermosetting resins, epoxy and unsaturated polyester, the epoxy resins have proved to be amenable to toughening by low levels of either liquid carboxyl-terminated butadiene acrylonitrile copolymer (CTBN) or amino-terminated butadiene acrylonitrile copolymer (ATBN). These liquid rubbers are effective in improving the crack resistance and impact strength while minimally effecting the heat distortion properties of the normally brittle epoxy resins. The enhancement in crack resistance and impact strength is brought about by the formation of a discrete rubbery phase during the curing of the epoxy resin. The size of the particles that constitute this phase is usually between 0.1 and 5 xcexcm.
Unfortunately, there are several problems associated with terminally-functional LRs. One problem is that end-functionalized polymer chains of liquid rubbers tend to react and crosslink, thereby increasing molecular weight, viscosity, and reducing miscibility. This problem is particularly severe among polymers that have reactive functional groups at each end of the polymer chain. Another problem is that the presence of terminal functional groups cause the LR to react prematurely with the liquid thermoset resins prior to cure. This also causes viscosity increases and/or reduced miscibility (phase separation) of the LR/thermoset resin liquid blend which makes processing difficult. Similar problems with increased viscosity also result from strong interactions between end-functionalized polymer chains and reactive groups on the thermosetting resins.
Another problem is that while CTBN and ATBN LRs are available for preparing miscible LR-modified epoxy liquid thermoset resins, no such LR is presently known that is both miscible in the uncured state and immiscible in the cured state with unsaturated polyester (xe2x80x9cUPxe2x80x9d) and epoxy vinyl ester thermoset resins. Unlike the epoxy resins, the incorporation of low levels of CTBN and/or ATBN LRs into unsaturated polyester resins results in negligible improvement in crack resistance and impact strength at the expense of reducing the heat distortion characteristics of the cured resin matrix.
The aforementioned problems thereby preclude the use of such blends, especially those based on unsaturated polyester thermoset resins, in processing operations that require low viscosities, such as pultrusion, resin transfer molding, and spray-up.
Moreover, when preparing LR/thermoset liquid resin blends, the end-user must carefully measure and mix these individual components. This hinders the preparation of xe2x80x9cone-packxe2x80x9d LR/thermoset liquid resin blends.
In view of the above problems, the composites and thermoset resin industry would greatly welcome the preparation of LRs and LR/thermoset liquid resin blends that: (a) remain miscible in the uncured state over time; (b) provide low viscosity and easy processability; (c) are chemically stable; and (d) toughen the cured thermoset resin with minimal decrease in heat and dimensional stability. The composites and thermoset resin industry would especially welcome the development of a LR that overcomes these problems in unsaturated polyester thermoset resins.
Accordingly, one object of the present invention is to provide LRs that: (a) remain miscible in uncured thermosetting liquid resins over long periods of time; (b) provide low viscosity and easily processable liquid resin thermoset blends; (c) are chemically stable; and (d) phase separate during cure for toughening the cured thermoset resin, which overcome the aforementioned problems.
Another object of the present invention is to provide efficient processes for preparing LRs that overcome the aforementioned problems.
Another object of the present invention is to provide uncured liquid thermoset resin blends, especially those based on unsaturated polyesters, with LRs that overcome the aforementioned problems.
Another object of the present invention is to provide composite materials made by curing liquid thermoset resin blends that overcome the aforementioned problems.
These and other objects, which will become readily apparent to those skilled in the art after reading this specification, have been accomplished by inventing new liquid rubber compositions that comprise polymer chains having a glass transition temperature less than 25 C. and at least one non-functional aromatic terminal end-group. We have heretofore discovered that liquid rubbers that are ordinarily immiscible in liquid thermoset resins can be made miscible by the addition of at least one non-functional aromatic end-group to the polymer chains of such liquid rubber compositions. In addition, these new liquid rubber compositions can be controlled to phase separate into rubbery microdomains upon curing of thermoset resins, including unsaturated polyester thermoset resins. The resulting new composite materials are found to improve the fracture toughness of cured thermoset resins while maintaining dimensional and heat resistance.
In a first embodiment of the present invention, there is provided a single phase uncured thermoset composition, comprising:
a) at least one uncured thermoset resin; and
b) at least one liquid rubber comprising polymer chains having a glass transition temperature less than 25 C., said polymer chains having at least one non-functional aromatic terminal end-group.
In a second embodiment of the present invention, there is provided a polymeric composition for cured thermoset resins, said polymeric composition comprising:
polymer chains having a glass transition temperature less than 25 C., said polymer chains comprising at least one non-functional aromatic terminal end-group;
wherein said polymeric composition is both miscible in the uncured state of said thermoset resins and immiscible in the cured state of said thermoset resins.
In a third embodiment of the present invention, there is provided a process for preparing a liquid rubber composition for a cured thermoset resin, wherein said liquid rubber composition is characterized as being miscible in the uncured state of said thermoset resin and immiscible in the cured state of said thermoset resins, said liquid rubber composition comprising polymer chains having at least one non-functional aromatic terminal end-group, a weight average molecular weight of at least 500 g/mol, a glass transition temperature less than 25 C., the process comprising the steps of:
(a) forming a reaction mixture, said reaction mixture comprising:
(i) from 1.0 to 99.999 weight percent of the reaction mixture of at least one C1-C20 alkyl acrylate monomer; and
(ii) from 0.001 to 20 weight percent of the reaction mixture of at least one aromatic-containing initiator;
(b) charging the reaction mixture into a reactor maintained at a reaction temperature up to 400 C.;
(c) initiating polymerization of said monomer with said aromatic-containing initiator to form a reaction product containing said polymer chains; and
(d) providing a reaction residence time sufficient for the polymer chains to form.
In a fourth embodiment of the present invention, there is provided a process for preparing a liquid rubber composition for a cured thermoset resin, said liquid rubber composition is characterized as being miscible in the uncured state of said thermoset resin and immiscible in the cured state of said thermoset resins, said liquid rubber composition comprising polymer chains having at least one non-functional aromatic terminal end-group, a weight average molecular weight of at least 500 g/mol, a glass transition temperature less than 25 C., the process comprising the steps of:
(a) forming a reaction mixture, said reaction mixture comprising:
(i) from 1 to 99.999 weight percent of the reaction mixture of at least one C1-C20 alkyl acrylate monomer; and
(ii) from 0.001 to 99 weight percent of the reaction mixture of at least one aromatic-containing solvent;
(b) charging the reaction mixture into a reactor maintained at a reaction temperature within the range of from 200 C. to 500 C.;
(c) thermally-initiating polymerization of said monomer to form a reaction product containing said polymer chains; and
(d) providing a reaction residence time sufficient for the polymer chains to form.
In a fifth embodiment of the present invention, there is provided a composite material, comprising:
a) at least one cured thermoset resin matrix; and
b) rubbery domains dispersed in said matrix, said rubbery domains comprising:
polymer chains having a glass transition temperature less than 25 C. and having at least one non-functional aromatic terminal end-group.
In a sixth embodiment of the present invention, there is provided a process for preparing single phase uncured thermoset composition, the process comprising the steps of mixing:
a) at least one uncured thermoset resin; and
b) at least one liquid rubber, said liquid rubber comprising polymer chains having at least one non-functional aromatic terminal end-group, said polymer chains having a weight average molecular weight of at least 500 g/mol, and said polymer chains characterized as having a glass transition temperature less than 25 C.